单项选择题

Cancer should be considered to be a biological problem rather than a medical one because （）

A. viruses are known to cause cancer in animals
B. at present, human cancer is not believed to be contagious(传染的)
C. there are many known causes for the transformation of a normal cell to a cancer cell
D. results of experiments on plants and animals do not vary greatly from species to species

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单项选择题

What is main purpose of the passage（）

A. To outline contrasting types of economy.
B. To explain the science of economics.
C. To argue for the superiority of one economics.
D. To compare barter and money exchange markets.

单项选择题

Why is the government asking for comments on the new rules（）

A. Because it is trying to fool people.
B. Because it doesn’t know how to make rules.
C. Because it wants to be nice.
D. Because it wants to hear what the citizens have to say.